1. Field of the Invention
The present invention relates in general to automatic transmissions of motor vehicles, and more particularly to a control for a frictional element of the transmission that is engaged when an associated motor vehicle is subjected to an engine brake.
2. Description of the Related Art
Hitherto, various automatic transmissions have been proposed and put into practical use in the field of wheeled motor vehicles. Some of them are of a type that establishes a desired speed (or desired shift stage) by engaging or disengaging clutches and brakes (viz., frictional elements) and has two operation modes that are switchable, one being a so-called engine brake operation mode (viz., a manual shift mode in which the shift is made manually) and the other being a so-called non-engine brake operation mode (viz., an automatic shift mode wherein the shift is made automatically).
In the automatic transmission of the above-mentioned type, when, under a shift operation of the manual shift mode toward a certain speed, a change to the automatic shift mode is intended and at the same time a shift instruction of the automatic shift mode does not match with the certain speed that has been aimed by the manual shift mode, it inevitably occurs that the frictional element (or clutch) for the engine brake that is about to be engaged is instantly released. However, in this case, interruption to an engine brake inevitably takes place, which makes the passengers in the vehicle, particularly the driver, feel uncomfortable.
For solving the above-mentioned drawback, it may be thought out that upon completion of shift in the manual shift mode, the automatic shift mode is forced to take place for executing a subsequent shift.
However, since the automatic shift mode is of a control that is not aimed for the engine brake, the mode is not provided with a data for controlling a hydraulic pressure of the frictional element used for the engine brake. Thus, upon switching to the automatic shift mode, the hydraulic pressure fed to the frictional element is instantly reduced thereby inducing an undesirable feeling of instant lack of engine brake. Furthermore, due to such instant reduction in hydraulic pressure, a shift shock is produced when a subsequent speed is about to be established.
For solving the above-mentioned drabacks, Japanese Laid-open Patent Application (Tokkaihei) 9-89092 and Japanese Laid-open Patent Application (Tokkai) 2003-314677 propose respective measures.
In the measures of the former Laid-open Application, when, under a shift operation from a first speed (where the frictional element for engine brake is engaged) to a second speed (where the frictional element for engine brake is disengaged), both a release instruction to the frictional element and a shift instruction to a third speed (where the frictional element for engine brake is engaged) are issued by a switching operation by the driver, the release operation for the frictional element for engine brake is suppressed to keep the existing condition and a shift from the first speed to the second speed and then a shift from the second speed to the third speed are carried out in order.
In the measures of the latter Laid-open Application, when, under a shift operation from a second low range L2 (where the frictional element for engine brake is engaged) to a first low range L1 (where the frictional element for engine brake is engaged), a so-called multiple shift instruction is issued for the change from the first low range L1 to the second low range L2, the engaged condition of the frictional element (viz., B-4 brake) for engine brake in the first speed is slowly released (viz., sweep down), and upon completion of the release operation, the shift from the first low range L1 to the second low range L2 is carried out and at the same time the frictional element (viz., B-2 brake) for engine brake in the second speed is slowly engaged.